In a 5- to 6-quart pan over high heat, bring 4 quarts water to a boil. Add pasta and cook, stirring occasionally, until tender to bite, 12 to 15 minutes. Drain.
Meanwhile, in a 12-inch frying pan or a 4- to 5-quart pan over medium-high heat, melt butter. Add onion and stir often until lightly browned around the edges, 8 to 10 minutes.
Lower heat to medium and add garlic, chili flakes, and crumbled chicken sausages. Stir with a wooden spoon, breaking up sausage if necessary, until meat is beginning to brown, 5 to 10 minutes; if garlic begins to scorch, lower heat.
Add tomatoes and spinach; stir until spinach is wilted, about 2 minutes.
In a large bowl, mix pasta and sauce to coat; top with ricotta salata.